Equilibrium binding studies of mono, di, and triisocyanide ligands on Au powder surfaces
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Abstract
Our group has previously shown that isocyanides are readily adsorbed from solutions to Au powder and bind to the Au surface in an end-on fashion through the terminal carbon. Later work demonstrated that the equilibrium constants for the reversible adsorption (eq 1) of electronically inequivalent isocyanides could be obtained using the Langmuir isotherm technique. This Au(s) + RNC ⇌ Au(s)/(RNC) (1)dissertation describes two projects completed which complement the initial findings of this group;Initially, several alkylisocyanides (RNC = n-C4H9NC, n-C6H13NC, n-C8H17NC, n-C12H25NC, and n-C18H37NC) were synthesized to examine the effect of tail length on Au powder adsorption. It was observed that the length of the alkyl chain affected not only the Au surface binding affinity, but also the rate of surface saturation and saturation coverage values. Direct competition studies were also studied using a 13C-labeled isocyanide (n-C18H37N13C). These studies demonstrated the stabilization afforded by substrate-substrate packing forces in SAM's formed by the longer chain isocyanides;In a second study, di and triisocyanides were synthesized to determine the effect that the length of the connecting link (i.e. # of (-CH2-) groups) and the number of isocyanide groups (as points of attachment) have on Au adsorption stability. Reports have shown that diisocyanides with flexible backbones such as C≡N-(CH2)6-N≡C and C≡N-(CH2)12-N≡C bind to Au through both -N≡C groups producing a SAM with a smaller elipsometric thickness than a diisocyanide with an inflexible backbone such as 1,4-phenylene-diisocyanide. Our work in this area describes the binding modes, relative binding affinities (Qab) and surface coverage values for a series of flexible alkyl and xylyldiisocyanides on Au powder surfaces.